Re: [R] A problem with anova()

From: Peter Dalgaard <p.dalgaard_at_biostat.ku.dk>
Date: Fri, 09 May 2008 09:42:27 +0200

Guru S wrote:
>
> Hi,
>
> I fitted tree growth data with Chapman-Richards growth function using nls.
>
> summary(fit.nls)
>
> Formula: HEIGHT ~ A * (1 - exp(-B * AGE))^C
>
> Parameters:
>
> Estimate Std. Error t value Pr(>|t|)
> A 29.007627 0.270485 107.24 <2e-16 ***
> B 0.030813 0.001095 28.13 <2e-16 ***
> C 1.849405 0.068659 26.94 <2e-16 ***
> ---
> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
>
> Residual standard error: 1.879 on 713 degrees of freedom
>
> Algorithm "port", convergence message: relative convergence (4)
>
> When I try to run the anova() function I get this:
>
> anova(fit.nls)
> Error in anova.nls(fit.nls) : anova is only defined for sequences of "nls" objects
>
> Could you tell me what the problem is?
>
>
It's what the message says: You can't run anova() on one model, only compare several, as in
anoval(fit.nls, fit2.nls).

It is not clear what anova(fit.nls) should do in the nonlinear case , since you cannot in general remove parameters successively from the model (notice, for your own model, that C is meaningless if B=0 and both B and C are meaningless if A=0).

-- 
   O__  ---- Peter Dalgaard             ุster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark      Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard_at_biostat.ku.dk)              FAX: (+45) 35327907

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Received on Fri 09 May 2008 - 07:44:49 GMT

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